The present invention relates to electrostatic inkjet print technologies and, more particularly, to printheads and printers of the type such as described in WO 93/11866 and related patent specifications.
Electrostatic printers of this type eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates an electrophoretic force that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location.
The location from which ejection occurs is determined by the printhead geometry and the position and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particle so that the direction of the electrophoretic force is towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and then ejection occurs at a highly localised region around the tip of the protrusions.
To operate reliably, the ink must flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoise. The material that is ejected is more viscous because of the concentration of particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will not spread significantly upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 01/30576 and WO 03/101741, all of which relate to the so-called Tonejet® method described in WO 93/11866.
FIG. 1 is a drawing of the tip region of an electrostatic printhead 1 of the type described in this prior art, showing several ejection upstands 2 each with a tip 21. Between each two ejection upstands is a wall 3, also called a cheek, which defines the boundary of each ejection cell 5. In each cell, ink flows in the two pathways 4, one on each side of the ejection upstand 2 and in use the ink meniscus is pinned between the top of the cheeks and the top of the ejection upstand. In this geometry the positive direction of the z-axis is defined as pointing from the substrate towards the printhead, the x-axis points along the line of the tips of the ejection upstands and the y-axis is perpendicular to these.
FIG. 2 is a schematic diagram in the x-z plane of a single ejection cell 5 in the same printhead 1, looking along the y-axis taking a slice through the middle of the tips of the upstands 2. This figure shows the cheeks 3, the ejection upstand 2, which defines the position of the ejection location 6, the ink pathways 4, the location of the ejection electrodes 7 and the position of the ink meniscus 8. The solid arrow 9 shows the ejection direction and also points towards the substrate. Each upstand 2 and its associated electrodes and ink pathways effectively forms an ejection channel. Typically, the pitch between the ejection channels is 168 μm (this provides a print density of 150 dpi). In the example shown in FIG. 2 the ink usually flows into the page, away from the reader.
FIG. 3 is a schematic diagram of the same printhead 1 in the y-z plane showing a side-on view of an ejection upstand along the x-axis. This figure shows the ejection upstand 2, the location of the electrode 7 on the upstand and a component known as an intermediate electrode (10). The intermediate electrode 10 is a structure that has electrodes 101, on its inner face (and sometimes over its entire surface), that in use are biased to a different potential from that of the ejection electrodes 7 on the ejection upstands 2. The intermediate electrode 10 may be patterned so that each ejection upstand 2 has an electrode facing it that can be individually addressed, or it can be uniformly metallised such that the whole surface of the intermediate electrode 10 is held at a constant bias. The intermediate electrode 10 acts as an electrostatic shield by screening the ejection channel from external electric fields and allows the electric field at the ejection location 6 to be carefully controlled.
The solid arrow 11 shows the ejection direction and again points in the direction of the substrate. In FIG. 3 the ink usually flows from left to right.
In operation, it is usual to hold the substrate at ground (0 V), and apply a voltage, VIE, between the intermediate electrode 10 and the substrate. A further potential difference of VB is applied between the intermediate electrode 10 and the electrodes 7 on the ejection upstand 2 and the cheeks 3, such that the potential of these electrodes is VIE+VB. The magnitude of VB is chosen such that an electric field is generated at the ejection location 6 that concentrates the particles, but does not eject the particles. Ejection spontaneously occurs at applied biases of VB above a certain threshold voltage, VS, corresponding to the electric field strength at which the electrophoretic force on the particles exactly balances the surface tension of the ink. It is therefore always the case that VB is selected to be less than VS. Upon application of VB, the ink meniscus moves forwards to cover more of the ejection upstand 2. To eject the concentrated particles, a further voltage pulse of amplitude VP is applied to the ejection upstand 2, such that the potential difference between the ejection upstand 2 and the intermediate electrode 10 is VB+VP. Ejection will continue for the duration of the voltage pulse. Typical values for these biases are VIE=500 volts, VB=1000 volts and VP=300 volts.
The voltages actually applied in use may be derived from the bit values of the individual pixels of a bit-mapped image to be printed. The bit-mapped image is created or processed using conventional design graphics software such as Adobe Photoshop and saved to memory from where the data can be output by a number of methods (parallel port, USB port, purpose-made data transfer hardware) to the printhead drive electronics, where the voltage pulses which are applied to the ejection electrodes of the printhead are generated.
One of the advantages of electrostatic printers of this type is that greyscale printing can be achieved by modulating either the duration or the amplitude of the voltage pulse. The voltage pulses may be generated such that the amplitude of individual pulses are derived from the bitmap data, or such that the pulse duration is derived from the bitmap data, or using a combination of both techniques.
The ejection characteristics of an electrostatic inkjet printhead are dependent on the geometry of the ejectors and on the positions of the electrodes at the ejector. Variation in these factors can lead to a variation in optical density or colour across a print.
The problem to be solved is to produce improved and more uniform ejection performance from an electrostatic inkjet print system whose raw performance produces a stable pattern of variation across the printhead. Prior knowledge of the characteristics of this variation enables the response of the print system to be calibrated to improve the uniformity of performance from the printhead significantly.
Electrostatic inkjet printheads can be controlled using the duration and/or amplitude of electrical pulses to the printhead ejectors to modulate the ejection from the ejectors. Unlike piezo or thermal inkjet printheads, in which the size of droplet ejected is primarily a function of the physical dimensions of the pressure chamber and nozzle, the volume of ink ejected from an electrostatic printhead ejector can be controlled by the amplitude and/or the duration of the electric field acting on the ink in the ejector, which in turn is determined by the voltage waveform applied to the electrodes of the printhead. This enables compensation for stable variations in the ejection performance across an array of ejectors to be achieved.
The ways in which the pulse duration and amplitude can be controlled are shown schematically in FIGS. 4A & 4B.
The volume of ink ejected in response to an applied voltage pulse is governed by the position of the ink meniscus, the electric field acting upon the ink and the duration of the applied pulse as described above. Ideally, every ejector in the printhead will perform equally, that is, will eject the same volume of ink at the same time for the same applied pulse. However, variation in ejector geometry, electrode positions or meniscus position across the printhead will cause variations in performance of ejectors leading to variation in the optical density of print across the width of the printhead. Such variation generally manifests as a gradual bow in print density from one side of the head to the other, is stable and characteristic of an individual printhead. As such, it can be compensated by choosing a set of pulse voltages and/or durations individually for each ejector or small groups of contiguous ejectors that equalises the print performance across the printhead. The calibration process both equalises the performance across the printhead and calibrates the tone reproduction curve (optical density versus image grey level) of the printhead in a single process.
Additionally, the response of the ink to an applied voltage pulse at an ejector is dependent upon the bias electric field (i.e. the electric field created by the application of the bias voltage to the ejector between ejections). In practice, the bias voltage VB is set just below the voltage VS at which spontaneous ejection occurs. It is important that VB is held close to VS (in practice about 20V below it) for the ink to respond rapidly to an ejection pulse. However, variations described above in ejector geometry and electrode positions can give rise to variation in VS across the printhead and consequently variation in the response of an ejector dependent on its position across the array.
US2006/018561 discloses a printer which adjusts for any variation in performance across the printhead by altering the pattern of dots which are needed to make up an image, thereby creating a new image, and then carrying out a standard transformation of that new image data into standard drive pulse values and hence into uncalibrated dot sizes. The calibration is achieved by creating a series of test prints for each channel in the printhead (see FIG. 8), so that the image data itself is calibrated rather than the ejected volume.
US2011/0234677 discloses a method of compensating for banding that occurs when a scanning printhead takes several interleaved passes to build up an image. Dark and light lines can result from errors in jet size and/or angle, and can result from the juxtaposition of certain nozzles on different passes, which don't have a one-to-one correspondence with individual nozzles. Hence, US2011/0234677 teaches making adjustments to the image (see FIG. 8) to compensate for banding in the print that is printed with a known interleaving scheme, which develops a characteristic pattern of banding from a given printhead. The correction would have to be re-done if a different interleaving scheme was used even for the same head. It specifically does not calibrate individual printhead channels by modification of print pulse values, but rather creates new image data which is then transformed into drive signals in a standard manner.
WO2012/040424 discloses colour profiling inkjet printing onto clear film. It involves printing a test pattern comprising greyscale patches, measuring the density of the greyscale patches, and adjusting output pixel values based on deviations between the expected and actual densities, all of which is well known colour profiling to achieve desired tone reproduction curves. WO2012/040424 teaches that the modification of pixel values is applied to the greyscale image before the image is then subjected to half-toning (screening to a small number of fixed dot sizes). This method does not carry out any dot size control (i.e. there is no control to the ejected volume to achieve a desired dot size) and as such, does not perform a correction of the printed dot sizes, but rather creates new image data which is then transformed into drive signals in a standard manner.